June 12, 2012 -- Another major study is pointing to significant increases in radiation exposure from the growing use of medical imaging tests such as CT scans.

For the latest study, which is published in the Journal of the American Medical Association, researchers combed through the medical records of millions of patients enrolled in six large HMOs around the U.S. They found that the percentages of patients who received high or very high doses of radiation from medical imaging tests roughly doubled over the last 15 years.

Researchers say the increases in radiation exposures seen in the new study were driven by a sharp uptick in the number of CT scans ordered for patients.

From 1996 to 2010,
CT scan rates tripled at the HMOs, rising from 52 per 1,000 patients to 149 for every 1,000 patients.

The increase was especially surprising in a population of patients treated at HMOs. Similar increases in medical imaging tests have been documented among Medicare recipients and in the general population -- where doctors get paid for every scan they order. But HMOs use a different financial model. That means doctors are ordering more imaging tests even when there's no financial incentive to do so.

Experts say the new study shows the reasons for the rise may be more complex than had been previously realized and that greater education about the dangers of radiation may be needed to curb testing where it has become excessive.

"We should make ourselves aware as providers, make our patients aware, and make our referring physicians aware that there are risks to the population from radiation," says Bibb Allen, MD, vice chairman of the American College of Radiology. "We certainly think that the benefits of imaging outweigh the risks. But that doesn't mean we should ignore the risks," says Allen, who was not involved in the research.

Concerns Over CT Scans

A CT scan, which stands for computed tomography, combines X-ray imaging techniques with computer software to create multiple cross-sectional images of the body. CT images give a more detailed look at internal structures of the body. This can identify abnormalities and also help guide doctors with procedures. But CT scans also expose people to radiation doses that are 50 to 500 times higher than the dose delivered by a typical chest X-ray.

"We do know that the higher dose will be associated with a much higher risk of cancer," says researcher Rebecca Smith-Bindman, MD, a radiologist and professor in residence at the University of California at San Francisco.

Although the current study did not look at the health effects of radiation, another one published last week in The Lancet found that children who have repeated CT scans before age 15 are at higher risk of brain tumors and leukemia.

Smith-Bindman says the risk of getting cancer from any one scan appears to be very small. But over time, doses may accumulate, increasing the risk. She says they will be looking at this issue in a future study.

The study also showed that doses of radiation delivered to patients can vary widely for the same test, depending on where it's performed. "Very few facilities know the doses they're using. They don't collect them. They don't look at them, so that's a huge problem," she says.

Though radiologists sometimes use higher doses of radiation to get a clearer image, Smith-Bindman says doses have escalated beyond the range that's needed for image clarity. "The doses are so much higher than they need to be," she says.

And researchers say that they found some evidence that testing was being ordered when it might not be needed.

"We know that in a lot of the cases, the CT really isn't necessary and maybe you can wait. Or if the CT is necessary, then maybe you can just go lower dose," Diana L. Miglioretti, PhD, tells WebMD. She is a senior investigator and biostatistician at the Group Health Research Institute in Seattle.

Miglioretti says the doses of radiation they documented in the study were chilling when viewed in the light of the risks of brain tumors and leukemia documented in the recent Lancet paper.

Assuming typical average doses for CT scans, The Lancet study found that having two to three CT scans of the head before age 15 exposed children to radiation levels that could triple a child's risk of having a brain tumor.

"What we found is that for 10% to 20% of kids, just one head CT scan got them to those levels. So that's really scary," Miglioretti says.

By 2010, 2.5% of patients at HMOs received high annual doses of radiation from CT scans, up from 1.2% in 1996. Similarly, the number of patients who got very high doses of radiation from CT scans jumped from 0.6% to 1.4% in the same period. A high dose was anything between 20 and 50 millisieverts (mSv). A very high dose was over 50 mSv.

The risk for getting higher doses of radiation increased with age. Older patients were scanned more frequently than younger patients.

What Patients Can Do

Researchers say patients shouldn't be shy about asking questions when doctors order a scan, especially if it's a CT scan.

"We need to start getting into these discussions," Smith-Bindman says.

Good things to know before you get a scan include why the test is being ordered, whether or not the test uses radiation, and if it does, how much radiation.

In some cases, she says, patients may have to call the radiologist or imaging clinic to find out about dosing since her study found that many doctors aren't aware of the radiation dose patients are getting.

If you get repeated scans, it's also a good idea to keep track of each test and its associated radiation dose.

SOURCES: Smith-Bindman, R. The Journal of the American Medical Association, June 12, 2012. O'Connor, G. The Journal of the American Medical Association, June 12, 2012. Rebecca Smith-Bindman, MD, professor in residence, departments of radiology, epidemiology/biostatistics, and obstetrics, gynecology, and reproductive medicine, University of California, San Francisco. Diana L. Miglioretti, PhD, senior investigator, biostatistician, Group Health Research Institute, Seattle. Bibb Allen, MD, vice chair, American College of Radiology, radiologist, Birmingham, Ala.